[Keyboard] Added Atreus Pro Micro variant (#8059)

* new alternative controller.

* following suggestions at qmk's 8059.

* forgot to replace some underscores and keycodes.

* following drashna's suggestions at qmk's 8059.

* following zvecr's suggestion in qmk's 8059.

* following noroadsleft's suggestion at qmk's 8059.

keyboards/atreus/atreus.h

@@ -24,6 +24,8 @@
     #include "astar_mirrored.h"
 #elif KEYBOARD_atreus_teensy2
     #include "teensy2.h"
+#elif KEYBOARD_atreus_promicro
+    #include "promicro.h"
 #endif
 
 // This a shortcut to help you visually see your layout.